This invention relates generally to semiconductor memories.
Phase change memory devices use phase change materials, i.e., materials that may be electrically switched between a generally amorphous and a generally crystalline state, as an electronic memory. One type of memory element utilizes a phase change material that may be, in one application, electrically switched between generally amorphous and generally crystalline local orders or between different detectable states of local order across the entire spectrum between completely amorphous and completely crystalline states.
Typical materials suitable for such an application include various chalcogenide elements. The state of the phase change materials is also non-volatile. When the memory is set in either a crystalline, semi-crystalline, amorphous, or semi-amorphous state representing a resistance value, that value is retained until reprogrammed, even if power is removed. This is because the programmed value represents a phase or physical state of the material (e.g., crystalline or amorphous).
Conventional sensing circuitry for thin film memories with an cyanic threshold switch (OTS) select switch and a memory element in series may use a fixed reference voltage or current that compares and discriminates cell state from the column voltage or current reached by accessing the cell. The read window is the voltage differential resulting, for example, from forcing current into a less resistive bit versus the voltage resulting from forcing current through a more resistive bit. The margin is basically determined by the worst case bit relative to (nearest) sense amp trip point within a group or block of cells. For example, a less resistive reset bit in series with a lower OTS hold voltage or a more resistive set bit in series with a higher OTS hold voltage may adversely affect the size of the read window, reducing margin, yield, and field reliability. Variation of threshold and hold voltages for the select device of the cells being read within a given group or block subtracts from the read window.
Thus, there is a need to make the read window or the read margin more independent of the bit-to-bit variations across a given array, group, or block of memory cells.